Fig. 7. Ultrastructure of putative release sites for neurotransmitters/neurohormones associated with the salivary gland. (A,B) Axonal profiles embedded between C-cells (asterisks) contain numerous clear vesicles and a few dense vesicles. Small electron-dense areas on the cytoplasmic face of the plasma membrane (white arrowheads) may represent active zones. (C) A putative release site on the outer surface of an acinar lobule, abutting a P-cell (white asterisk). A thick basal lamina (black asterisk) separates the presynaptic membrane from the plasma membrane of the P-cell. (Insets) Ribbon-like electron-dense structures (arrowheads) on the cytoplasmic face of the axonal membrane at higher magnification, in cross-section (left) and en face view (right). Rows of vesicles are adjacent to these electron-dense structures, suggesting that they represent the structural correlates of active zones. (D–F) Axonal profiles within nerves that interconnect acinar lobules. On one side, the axonal profiles are without glial wrapping, face a thick basal lamina (asterisks) and have electron-densities on the cytoplasmic face of their plasma membrane (arrowheads). Type A release sites (D,F) have numerous small clear vesicles and few larger dense vesicles. Type B release sites (E) contain fewer clear vesicles and numerous dense vesicles. Scale bars, 0.5 µm (A–F); 0.2 µm (inset).